Microbial Technologies For Converting Phosphate-Loaded Fe(III) Oxides Into Useful Agricultural Fertilisers

Abstract

As the global population increases, the demand for food will rise. To ensure food security, there is interest in developing novel and sustainable approaches to crop fertilization. Phosphorus (P) is one of the major nutrients needed for the growth and development of plants. However, the phosphorus used in the fertiliser industry is mined from phosphate rock reserves (a finite and non-renewable resource) using unsustainable mining methods with significant environmental impacts. Vivianite, a phosphate-containing Fe(II) mineral has been formed via microbial Fe(III) reduction in the presence of phosphate at neutral pH conditions, but there is limited information on systematically controlling the parameters that can maximize microbially-induced vivianite formation using synthetic ferrihydrite as a starting Fe(III) substrate. There is also interest in the use of phosphate-containing waste Fe materials as a source of Fe(III) for vivianite formation, offering opportunities for waste reuse, mediated by indigenous microbial communities in the waste, and overall, a relatively low-cost alternative to vivianite synthesis. For sustainable phosphorus recovery, vivianite which has been previously used as a Fe source to correct Fe chlorosis in plants, could act as a potential phosphorus source for plants. Only a limited number of studies to date have used synthetic vivianite as a phosphorus fertiliser, and there is no published research on the topic of using microbially-mediated vivianite for this purpose. This thesis has focussed on addressing these knowledge gaps and providing insights into maximizing vivianite production using Fe(III)-reducing bacteria, and the potential of using "biovivianite" as a plant fertiliser. Microbial Fe(III) reduction experiments were performed using synthetic ferrihydrite, ferric citrate and waste Fe residues as substrates for vivianite bioreduction, and solid bioreduced minerals were characterised using techniques including XRD, FTIR, SEM-EDX, XPS, and Mossbauer spectroscopy. Optimisation experiments identified the conditions for maximising the microbial synthesis of vivianite, with ferrihydrite reduction initiated at a pH below 7, with a P/Fe ratio of 1 in the presence of an electron shuttle and the use of the model metal-reducing bacterium Geobacter sulfurreducens. In these bioreduction conditions, vivianite was the dominant Fe(II) biomineral identified after a green rust intermediary phase. In experiments using waste Fe(III) residues, it was found that using wet non-agglomerated waste Fe residues would offer a sustainable and circular economy route to valorising waste Fe(III) materials and producing vivianite by using indigenous Fe(III)-reducing communities in the waste Fe(III) materials. For maximising vivianite formation in dried agglomerated waste Fe residues, it was found that additional phosphate amendment and long bioreduction times may be necessary. To finally test the bioreduced products as a fertiliser, three types of microbially-mediated vivianite materials (biovivianite) were produced using the starting Fe(III) substrates insoluble ferrihydrite or soluble ferric citrate, at pH 6.5 or 7.0. It was found that biovivianite produced with ferrihydrite at a starting pH of 7.0, containing vivianite, metavivianite and amorphous non-reduced ferrihydrite (28 % Fe(III)) was an effective Fe source to enhance the growth of white lupin. For phosphorus fertiliser efficiency, biovivianite produced with ferric citrate, containing phosphate green rust and vivianite, was equivalent to the application of 74% of soluble KH2PO4 compared with a chemically synthesized vivianite which was only 16%. In summary, the thesis gives valuable insights into the approaches to maximise vivianite production and for the first time shows the potential of microbially-produced vivianite as a phosphorus fertiliser for plants, providing an option for sustainable phosphorus recovery and reuse.

Date of Award	22 May 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Lloyd (Main Supervisor) & Victoria Coker (Co Supervisor)